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Fast Reactors, Gas Reactors, and Military Reactors
Published in Robert E. Masterson, Nuclear Engineering Fundamentals, 2017
Some research and test reactors also use another coolant called sodium–potassium (NaK) because it has a relatively low melting point. NaK has been used as a coolant in experimental fast reactors. Unlike commercial nuclear power plants, test reactors and research reactors have to be frequently shut down and refueled. Using conventional coolants like liquid lead or liquid sodium would require continually heating them to maintain them in a liquid state. The use of NaK overcomes this problem, and makes them easier to refuel. Liquid NaK, which has a potassium content between 40% and 90%, continues to be a liquid at even room temperatures (~30°C), and so it is very attractive to use when the cost of a reheating system becomes an issue. The Soviet RORSAT radar satellites used liquid NaK as their coolant. As well as having a wide liquid temperature range, NaK has a very low vapor pressure, which is very important in the vacuum of space. As we will soon discover, most fast gas reactors use helium as their coolant. It has a small absorption and scattering cross section, and therefore preserving the fast neutron spectrum without significant neutron absorption in the coolant. Various fast reactor coolants are compared to each other in a later section of this book.
Fast Reactors, Gas Reactors, and Military Reactors
Published in Robert E. Masterson, Nuclear Reactor Thermal Hydraulics, 2019
Some research and test reactors also use another metallic coolant called sodium–potassium (NaK) because it has a relatively low melting point. NaK has been used as a coolant in experimental fast reactors. Unlike commercial nuclear power plants, test reactors and research reactors have to be frequently shut down and refueled. Using conventional coolants such as liquid lead or liquid sodium would require continually heating them to maintain them in a liquid state. The use of NaK overcomes this problem and makes them easier to refuel. Liquid NaK, which has a potassium content between 40% and 90%, continues to be a liquid at even room temperatures (~30°C), and so it is very attractive to use when the cost of a reheating system becomes an issue. The Soviet RORSAT radar satellites used liquid NaK as their coolant, and in addition to having a wide liquid temperature range, NaK has a very low vapor pressure, which is very important in the vacuum of space. The final annoying feature of liquid sodium is that when it absorbs neutrons, and even fast neutrons, it becomes a copious beta and gamma emitter called Na-24. which has a half-life of 15 hours. Na-24 is highly radioactive, and because it reacts so violently with water, it is usually not advisable to allow this isotope to be pumped directly from the core to the steam generators. Hence, fast reactors cooled with liquid sodium use two sodium loops - a primary sodium loop containing this radioactive sodium and an intermediate loop containing non-radioactive sodium which carries the heat from the primary loop to the steam generators through an intermediate heat exchanger (or IHX). The IHX is then where the actual heat transfer between the two sodium streams occurs. Since the sodium in the primary loop is highly radioactive, all components in the primary loop must be heavily shielded. Also, because sodium reacts violently with the water vapor in ordinary air, most of the components in the primary loop must be immersed in an atmosphere of nitrogen or another inert gas, with which the sodium does not react. For similar reasons, empty regions above the sodium in the reactor pressure vessel and the coolant pumps must be filled with argon or another gas which is chemically inert. A combination of these practices has led to many successful fast reactors with few, if any, operational problems. In almost all of these reactors, liquid sodium is used as the coolant of choice.
Nuclear Security Considerations for Space Nuclear Power: A Review of Past Programs with Recommendations for Future Criteria
Published in Nuclear Technology, 2020
Past nuclear reactor systems designed for LEO operations include NERVA, Space Power 100 kW(electric) (SP-100), and the Soviet Buk used on the Radar Ocean Reconnaissance Satellite (RORSAT) missions. The nuclear systems designed without LEO operations or start-up include Jupiter Icy Moons (JIMO)/Prometheus and Kilopower, and the Russian systems Topaz I and Enisy (renamed Topaz II). The space mission for NTP has yet to be defined. For FPS and NTP systems, both LEO and non-LEO operations are being considered, but based upon a review of past projects, it is clear that non-LEO operation is the preferred mode from both safety and safeguards perspectives.